Subscribers of mobile communication networks of the second and third generation are offered services which require access to special network subsystems, as for example an IP Multimedia Subsystem (IMS).
The 3rd Generation Partnership Project (3GPP) has standardized the IP based multimedia subsystem for UMTS Release 5, cf. “3GPP TS24.228 V5.0.0 (2002-03); Technical Specification Group Core Network (Release 5)”. The system provides generic functionalities in order to establish sessions via packet switched access networks, e.g. General Packet Radio Service (GPRS).
IMS as a standard for next generation networks is based on the Session Initiation Protocol (SIP). It provides a control architecture which is able to handle the corresponding messages and sessions. Yet a number of services are based on more than one protocol. Already the first standardised service ever which makes use of IMS, the PoC service (Push-to-talk Over Cellular), includes communication based on the HTTP protocol between a client and the network.
However, IMS does not provide any means to handle protocols other than SIP.
IMS Standard Architecture
IMS consists of a number of two proxies and a registrar for SIP messages. The registrar is connected to a database which stores all information necessary to process a subscriber session in his subscriber profile.
FIG. 1 shows a standard IMS network architecture and the correlated standard IMS flows. A mobile client or subscriber sends a register message (SIP method) to a Proxy Call Session Control Function (P-CSCF) 10. The P-CSCF 10 is the initial interface (SIP Server) between the client and the IMS. The address of the P-CSCF 10 is discovered either as part of the Session Management procedures involved with establishing IP connectivity. That is, the address of the P-CSCF 10 may be provided during a PDP Context Activation process. The P-CSCF 10 forwards the register message to an Interrogating Call Session Control Function (I-CSCF) 12. The I-CSCF 12 is an IMS element that provides a contact point within an operator's network. Allowing subscribers of the network operator, or roaming subscribers to register. The I-CSCF 12 deals will registration, routing and forwarding of SIP messages and charging. The I-CSCF 12 selects the appropriate server function (registrar) that is a Serving Call Session Control Function (S-CSCF) 16 based on a response from a Home Subscriber Server (HSS) 14. The HSS 14 describes the many database functions that are required in next generation mobile networks. These functions include the Home Location Register (HLR), Domain Name Servers (DNS) and security and network access databases HSS. According to the response from the HSS 14 the message is then forwarded to the appropriate S-CSCF 16. The S-CSCF 16 is an IMS element that provides the session control services for a mobile client, which enables the network operator to support the services. Functions include: the management of mobile registrations, maintaining of the session, interaction with other services, charging and security.
The S-CSCF 16 downloads the credentials for authentication of the client from the HSS 14 and uses it, after successful authentication. It also downloads the subscribers profile from the HSS 14. (Response flow directions are not shown in the drawings).
The flow for session invocation is going from the client to the P-CSCF 10, but subsequently to the registrar, S-CSCF 16, which forwards the message to a required application server (AS) or recipient. This is shown in FIG. 2.
After receiving an initial request of a session from the client, the S-CSCF 16 looks up the subscribers profile for matching filter criteria (iFC) and performs the actions related to it in case of positive match.
The Cx interface connecting the I-CSCF 12 and the HSS 14 is the key element in using the subscriber profile for subscriber specific handling of SIP messages or forwarding the messages to specific server instances or B-parties.
At the moment IMS does not provide any means to handle protocols other than SIP.